This invention relates to a method for producing a glass article of a devitrifiable glass without occurence of any devitrification.
Generally, glass articles are produced by molding molten glass and then cooling the molded glass. Various molding methods have been known such as casting, blowing, drawing, swaging and rolling, and a suitable method is employed for an article to be molded.
The glass molding operation is effected at a temperature in the workable temperature range of the glass, and the optimum working temperature is determined by the composition of the glass to be used, the shape of the article and, therefore, the method employed.
In cooling the molded glass, devitrifications may disadvantageously develop if the molded glass is maintained for a considerable period at a certain temperature range below the liquidus temperature of the glass, which has been usually referred as a devitrifying temperature range.
If the working temperature in producing a glass article is within the devitrifying temperature range of the glass, devitrification may take place during the molding operation to cause loss of transparency, deterioration of the mechanical strength and deterioration of the commercial value of the product.
In order to avoid these disadvantages, the composition of glass has been so selected that the working temperature is sufficiently removed from the liquidus temperature to be outside the devitrifying temperature range.
There are, however, certain cases where such selection of the glass composition is not possible because of the properties of the glass article to be obtained.
A method for obtaining a glass article without devitrification of a glass having a working temperature within or close to the devitrifying temperature range is of great benefit.
Such a method is disclosed in Japanese Pat. Publication NO. 38519/1971. According to this method, a devitrifiable glass is prepared in the form of powdery glass and then such powdery glass is molded into a desired article by using an extrusion molding method at a temperature below the devitrifying temperature range. In this method, since the temperature of the glass is not elevated above the devitrifying temperature range during the producing process, there is no chance for devitrification to take place. This method, however, requires enlarged producing installations. Moreover, since the glass must be prepared in the form of powdery glass and then again heated and shaped, the producing steps are extremely complicated and pose an economical problem.